EF86/EL84-based 18W Prototype Amp


deus ex machina
Apr 27, 2012
I am finally ready to part with an EF86/EL84-based prototype that I built in 1997. This amp is a true prototype down to the aluminum chassis, which was hand sheared, folded, drilled, and punched. The eyelet boards were made from 1/8" LE-grade phenolic sheet. I started with the basic 1959 AC-15 circuit, which employed an EF86-based preamp instead of an ECC83/12AX7A preamp like the later models. I replaced the tube-rectified power supply with a silicon rectified power supply and beefed up the power supply capacitance. The circuit was then revoiced to remove the tubby bass found on the AC15. The result is akin to the difference between a JTM 45 and a 1987. The tone of the amp is more aggressive than the original circuit. The bass and treble controls are neither FMV or Hiwatt. The treble control is actually a reversed wired cut control. The bass control is fairly unique to this amp. It is more like a girth control in that it slowly adds more capacitance to the cathode bypass capacitor on the EF86, thickening the tone of the amp. The volume control is a post-phase inverter design. The PABE/PABC switch adds or removes local negative feedback from the power section by switching the power tube cathode bypass capacitor in (PABE) and out (PABC) of the circuit. PABE stands for power amp bandwidth expand while PABC stands for power amp bandwidth compress. The effect is more noticeable at higher volume settings. The amp is cathode biased and cathode biased amps tend to be more touch sensitive than fixed bias amps. Removing local negative feedback makes the power section even more squishy with a more mid-focused tone. There is a reduction in clean headroom in PABC mode.

Both the head cabinet and speaker cabinet were built to my specifications by Will Dyke at Armadillo Ampworks back when he was working on his back porch with a contractor saw and jigs/fixtures.. The cabinets are covered in Ampeg check tolex. The grille cloth is SVT grille cloth. The speakers are C10Qs that were built by Ted Weber Sr. back when he was still working out of his home. The speaker is now sold by his son as the 10F125.

This amp is available for free to any forum member who is willing to come pick it up in Annapolis, Maryland, as I will not ship the head and cabinet. I am downsizing to a significantly smaller abode and reducing the number of amps I own to one small amp. This amp had no control legend for most of its life. I applied P-Touch labels, so that the new owner would know what the controls and switches do. Anyone who is seriously interested in the amp and willing to pick it up locally should send a PM.




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Boy, if I was a state or two closer....

Very nice of you to do that. And very cool design! I'd love to rock that, but I'm sure you'll find a good home for it.
It is the last amp I built that I still own. The speaker and head cabinets are made from finger-jointed pine with the front and back panels on the head and the baffle board and back panel on the speaker cabinet made from Baltic Birch.
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There was no such thing as a kit guitar amp when I built this amp. The only off-the-shelf chassis one could buy was a Tweed chassis from the original owner of Mojo Tone and he did not sell retail. It was a very different time. While the boutique amp market has always been a cottage industry, it was even more so when this amp was built. It was a fun time with all of the JAN American surplus and Soviet-designed tubes flooding a market that had previously been slowly dying after the closure of Sylvania’s last tube factory in Emporium, PA in 1988. I am sure that the PRS Harmonic Generator was a response to the end of American tube production. Every major manufacturer was racing to come up with solid-state designs that could replace tube amps. Peavey came up with Transtube. Marshall’s answer was Valvestate.
I can be there Thursday evening. No, not really. Although I'm a little over an hour away, I have 3 amps I don't play through enough. There is a deserving person out there who can use and appreciate it.
Really cool of you.
I do know if anyone at the factory would be interested, but it is funny that you mention it. I took the amp over to John Ingram’s house shortly after I built it. I had John (a.k.a. Orkie) switch my Annapolis Shop Standard 24 over to McCarty wiring because I could not bring myself to use a reamer on the guitar. He was impressed enough to suggest that Paul would like the amp because it did not have the typical tubby AC-15 bass that Jeff Bober’s Twinmaster possessed (Budda was a young company at that point in time). Jeff is another local guy. He worked with father at Westinghouse before the Northrop Grumman acquisition. The early Budda amps were built at Precision Audio Tailoring in Glen Burnie, Maryland, which was Jeff’s amp repair shop. Anyway, I suspected that John’s suggestion would lead to an expectation of me giving the amp to Paul, which I was not prepared to do at that point in time.
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I cleaned all of the pots with DeoxIT yesterday evening. It was the first time I had the chassis outside of the shell in at least ten years. As you can see, the amp is not another Tweed or Blackface metoo design. I was trying to build something unique, something that I could not purchase.

Here is a gut shot:


I was kind of obsessed with Hiwatt-style lead dresss when I built this amp. All of the twisted leads other than heater wires running along the backside of the amp are post phase inverter. Most guitarists understand that a post-phase inverter master volume comes after the phase inverter, but many do not know that a cut control is a post-phase inverter tone circuit. A cut control takes advantage of the fact that the two signals coming off of a phase inverter are 180 degrees out of phase; therefore, they sum to zero. A cut control is a capacitor and potentiometer wired as a rheostat (a.k.a. variable resistor) that sets the frequency threshold above which the signals from the two phases sum. Using tightly twisted pair for these circuit leads eliminates the need to use shielded wire, as any stray voltages sum to zero, lessening the chance of oscillation occuring due to parasitic coupling. I used a four-diode bridge instead of an integrated bridge rectifier and laid it out like how a bridge rectifier appears on a schematic because I thought that it looked cooler in amp than an integrated bridge rectifier. The two resistors attached to the diodes are the heater hum circuit. Like most old school amps, the amp uses AC voltage for the heater circuit. Modern high-gain amps tend to use DC voltage for the heaters to reduce the amount of amp hum, at least for the preamp tubes. This amp has a very short signal path; therefore, hum reduction is not as critical.

With that said, unlike a 12AX7A, which is a dual triode, an EF86 is a small signal pentode. An EF86 has significantly more gain than one half of a 12AX7A, but not as much gain as both halves of a 12AX7A wired in series. However, most amps that employ 12AX7As wired in series tend to attenuate the output of each gain stage. The major difference between an EF86 and a 12AX7A is tone. An EF86 has more brilliance than a 12AX7A. It also overloads like a power tube, so putting Tube Screamer or derivative in front of an EF86-equipped amp is a different experience than a 12AX7A-equipped amp. It is why the early AC30/4 amps are so sought after. The Top Boost circuit was an attempt by Vox to capture the brilliance of the EF86 after they switched to using a 12AX7A-based preamp in the AC30. The downside to the EF86 is that they have to be selected for microphonics. The amp currently has a Mullard EF86 in it, which has a great tone, but one has to put up with microphonics. The RFT EF86 appears to be the least microphonic of the currently available EF86s. The reason why I changed V1 over from being a ceramic socket to being a plastic socket was to reduce vibration. Microphonics are the one downside to using an EF86, but if you want that tone, nothing else will do.
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Super clean build! Here is the last one I built. It's a "High Octane" from the AX84 site. SE EL84 power stage with a high gain Marshall type front end.